1. Field of the Invention
The present invention is concerned with methods for protecting copper-containing surfaces from corrosion, and is especially used for inhibiting corrosion of copper substrates that are to be used in printed circuitry that requires storage over appreciable periods of time prior to attaching other components by soldering. The present invention is also concerned with the protected copper products obtained by the methods of the present invention.
2. Background Art
Copper and its alloys are the most commonly used metals in electronic applications and especially in providing conductive circuit paths for printed circuit boards and printed circuit cards.
Very often it is necessary to store printed circuit boards and cards after their manufacture for substantial periods of time of up to a year or more before circuit elements and devices are inserted and soldered to the printed circuit board or card. Unfortunately, copper and copper alloys have a tendency to react with various agents present in the air such as oxygen, water vapor, carbon dioxide, chlorine and sulfur. This, in turn, results in a decrease in the solderability of the copper-containing material over a period of time. In order to prevent corrosion of these copper containing surfaces and to preserve its solderability, various procedures have been suggested. For instance, it has been suggested to plate solder onto the copper-containing surface within a relatively short period of time after the bare circuit board or card has been completed. Although this method has been found effective in preserving the solderability of the circuit boards and cards, this is a time-consuming and costly procedure which also suffers from the deleterious effect of contributing to a short circuit on fine-line, high-density printed circuits
Another common method employed to preserve solderability is the use of an immersion tin technique to protect the copper. However, this technique is relatively expensive and the tin is susceptible to corrosion by water vapor under relatively high humidity conditions.
In addition, there have been a number of suggestions for employing various azole materials to inhibit the corrosion of copper. For instance, U.S. Pat. Nos. 3,933,531; 4,134,959; 4,123,562; 4,373,656; 4,395,294; and 4,402,847 suggest various processes employing azoles for treating copper. However, treatments with azoles have not been entirely satisfactory since the extent of protection afforded has not been as good as the more expensive techniques such as the immersion tin technique.
The latest trends in the surface mount assembly processes have created the need for the use of compatible surface coatings for copper pads and plated-through holes (PTHs) which require protection from oxidation and corrosion until soldering operations are performed on the assembly line. These trends are: 1) Use of mixed (hybrid) technology; 2) Assemblies requiring multiple heat cycles, 3) Elimination of post-assembly cleaning operations; 4) Use of less active "no clean" and VOC-free pastes and fluxes; 5) High VO components (fine pitch); and 6) use of electrically-conductive adhesives in place of solders. To keep up with these transitions, the protective coatings used on copper surfaces must be replaced by material(s) which will satisfy all these demands and still be cost effective.
Benzotriazole and Hot Air Solder Levelling(HASL) are extensively used coatings in the electronic industry to protect copper surfaces and protect their solderability. However, benzotriazole cannot withstand multiple heat cycles and HASL, besides being expensive, produces thermal stresses in the card, resulting in warping or IP separation problems. HASL-treated cards also cannot maintain co-planarity of surfaces due to variations in the thickness and the crowning of the solder. The coating industry has responded to these problems by developing imidazole based coatings which can withstand multiple heat cycles, protecting the copper surface from oxidation. However, such coatings are thick (0.3 to 0.5 microns) and create problems in achieving a required degree of solderability or capillary action during wave soldering operations when used with less aggressive "no-clean" fluxes. Imidazole coatings may also create problems in the so-called "Bed of Nails" tests by forming a barrier to probes contacting the underlying copper surface or by coating the tips of the probes with insulative residues after repeated testing. Metallic coatings like immersion gold or palladium on the top of electroless nickel can overcome the insulating nature of thick organic coatings, but these are usually expensive due to the high precious metal costs and the slow plating processes. Rosin or resin-based coatings (prefluxes) will withstand multiple heat cycles but require solvent-cleanable solder pastes and fluxes, which are becoming outdated due to the drive for environmentally safe processes.
To meet the demands of advancing technology, it is clear that improved coatings and processes are needed which would combine the advantages of both organic and metallic coatings and optimize coating performance during the assembly of electronic components.